The Dell Latitude D600 does not officially support Windows 7, but it will work. Being an older system, performance will be noticeably reduced relative to a newer computer. Windows 8, 8.1, and 10 will NOT run on the D600, and Windows should not offer the Get Windows 10 update.

Installation Preparation
The stock specifications for the D600 are rarely able to meet the requirements for Windows 7, and it is more than likely you will need to upgrade the RAM and hard drive.

RAM
Most Latitude D600s have two sticks of 256MB or 512MB DDR memory, for a total of either 512MB or 1GB of system memory. While 1GB is enough to use Windows 7 and 512MB is enough to install, increasing the amount of memory to the maximum of 2GB RAM is optimal to ensure full performance. The D600 will accept two 1GB PC2700 DDR modules, which cost about $21.

Hard Drive
As with the D610, the Latitude D600 is usually limited by hard drive capacity. After Windows Update, roughly 30 GB of hard drive space will be used. The majority of D600s shipped with hard drives under or including 40 GB, and these will need to be upgraded to ensure there is room for documents, programs, and Windows updates. If you have a lot of data to store, the 60GB hard drive is likely not enough. The Latitude D600 uses 2.5" IDE hard drives, which are commonly available in 80GB capacity for about $25. I have not tested this behavior extensively, but drives over 120GB may have issues with the BIOS; Windows overrides the BIOS and can access large drives. Such large drives, as typical with legacy interfaces, tend to be significantly more expensive.

Processor
The Latitude D600 uses a 400 MT/s system bus, supporting the earlier Pentium Ms listed as having a 400 MHz bus. The processor is one of the slower system components, and unfortunately it does not have much of an upgrade route. The Pentium M inside is available in clock rates from 1.5 to 2.0 GHz, with compatible processors reaching 2.1 GHz. I recommend using a Pentium M 745, 755, or 765; unless you have the 1.5 GHz processor, you may not notice much of a difference in performance. The Celeron M will work in the D600, but performance will be worse than even the slower Pentium Ms.

Windows Installation
Due to hard drive capacity issues, I strongly suggest you perform a clean installation of Windows 7 unless the drive is larger than 80GB. Installation can be started from either an installation DVD or a USB thumbdrive. These instructions are for a clean installation, not preserving data -- upgrade installation varies significantly (though it is not nearly as difficult) and thus will not be covered in this tutorial.

Use the boot menu (F12) in the BIOS POST screen to boot to whatever boot device you are using to install Windows. After Windows loads its files, select Install Now, accept the EULA, and select Custom (Advanced) for the installation type. Delete existing partitions (your data disappears permanently after this!) and create a new partition with the default (maximum) size. Press 'Next' and Windows should begin to install. Let it do its thing, then follow the on-screen instructions until you arrive at the desktop.

Drivers
Due to the age of the system, most drivers are preinstalled with Windows 7.

Display
All Latitude D600s use ATI Radeon 9000 graphics. These are not capable of running the Aero interface, so you are forced to settle for the Basic theme. The driver is provided with Windows 7; simply set your display resolution to the native setting (either 1400x1050 or 1024x768) after installation if Windows does not do so already. Do not use the ATI drivers from Dell as it poses a compatibility issue.

Network
The Broadcom Ethernet drivers are preinstalled with Windows 7, but Wi-Fi may not. Should you find you need a wireless driver, the following drivers will work under Windows 7:

Bluetooth
I do not know if the Dell TrueMobile 300 is preinstalled with Windows 7, but I would expect it to be. If not, the Dell driver can be installed under Windows XP compatibility mode, but I advise against installing this driver unless doing so is necessary.

Modem
The Latitude D600 uses the Conexant D480 modem. Ideally, this driver is installed via Windows Update, but Dell's driver R104084 can also be used.

Audio
Like the Latitude D610, the D600 uses Sigmatel STAC 975X audio. As this audio chip uses the AC '97 bus, you will not be able to use certain features such as audio enhancements and jack detection. Dell provides this driver as R99254.

Smart Card Reader
The smart card driver is not provided by Windows 7. Use the Dell-provided driver R114084; it does not need to be run in compatibility mode.

Post-Installation
Disk space is going to be your biggest concern. After installation and Windows Update, run the Disk Cleanup utility and select "Clean up system files". Delete shadow copies and restore points (you shouldn't need them), then clean up the selected files. Manually delete the files in the %TEMP% folder.

Windows Update takes a long time to run (in the 12 hour range), and the system will be running at 100% CPU usage nonstop during this period. A patch was released to attempt to fix this, KB3102810. I have encountered issues with installing updates after installing this patch, but detection was fast and it allowed me to install drivers. If you have your system running in a 100% CPU usage state, ensure it is able to adequately cool itself. Clean your heatsink vents regularly, and monitor temperatures with a utility such as I8kfanGUI or CoreTemp. If you use I8kfanGUI, don't change the fan control mode unless you know what you're doing.